MIMO communications systems use multiple antennas for both transmitting and receiving to improve communication performance. In MIMO wireless communication systems, a transmission technique known as spatial multiplexing is used to transmit independent and separately encoded data signals (i.e., data streams) from each of the multiple transmit antennas. Decoding the received data at MIMO receivers can be challenging. As such, multiple decoding algorithms with different complexity-performance tradeoffs can be used.
The Maximum Likelihood Detector (MLD) Real Model is an example of a decoding algorithm that can be used at a MIMO receiver to detect transmitted symbols from a MIMO transmitter. To detect a transmitted symbol, an MLD tree may be generated and searched to find the node having the minimum Partial Euclidean Distance (PED). The node with the PED is selected as the estimate of the transmitted symbol from the received symbol.
Instead of searching the entire MLD tree, different schemes have been used to reduce the search space by imposing constraints. As such, different search strategies, such as, depth first and breadth first, have been used. One type of breadth first search strategy is a K-Best/QR decomposition (QRD)-M Tree Search scheme. The basic idea for this tree search scheme is at each level of the MLD tree, a decoder will sort and keep only K nodes with the minimum PED and discard the rest before proceeding to the next level. The decoder utilizing a K-Best search scheme always works in a feed-forward fashion. That is, when processing at a given level, it does not traverse back to any previous levels. At the next level, only the survived K nodes are expanded from the previous level till the leaf nodes are reached. The decoder then searches for the node with minimum PED among the leaf nodes resulting in a pseudo ML solution without the calculation complexity of the MLD.
Various versions of the K-Best strategy, therefore, have been used when detecting received data to reduce the number of calculations needed but still maintain sufficient results. However, in order to support higher order modulations such as 64-QAM or 256-QAM, a higher value of K is needed in order to get a Bit Error Rate (BER) performance that can approach that of MLD. For example, a higher value of K=16 can be used to support these modulations.